Everyone uses metaphors. Rightly so. They're a natural way to illustrate. Use them wrong, though, and they can really foul you up.
Do not want to get fouled up? Then know the 3 good rules about metaphors (and all other figures of speech, for these apply broadly).
1. Do not mix them. Saying something like "knowing the ropes paves the way for a fruitful harvest," for example, as I saw in a real memo once, is illogical. (Ropes, pavement, and agriculture have nothing to do with each other.) Why does logic matter? Because if you mix imagery like this, you'll rightly be accused of not thinking through what you're writing.
2. Do not set off your metaphor with "quotation marks" or the British 'inverted commas'. It's amateurish. Your reader is smart enough to know when you're using a figure of speech. You'd only use this punctuation if you were defining some unusual or made-up word. This is called a "neologism." Even with a neologism you'd only use quotation marks once, when you defined your new term; ever after, your reader would not need them. Neologisms are not usually metaphors, in any case. So just remember, no special punctuation for metaphors.
3. Make up your own metaphors. Do not use ones you've already heard. This is important. First, using someone else's one makes you look lazy, which you are. Second, because it's lazy, sooner or later you'll accidentally mix one, or you'll use one that is not quite right for the situation. And you'll lose your credibility. So never talk about needles in haystacks, or taking bulls by the horns, or anything else you've heard before. Invent new ones.
Here is some vocabulary to be clear about. These are three terms you'll hear from time to time, whenever people are talking about figures of speech. A metaphor, technically, is an implied comparison, such as talking about all world being a stage, and the people on it players. A 'simile' (pronounced SIM-uh-lee) is the same idea, only more obvious, and it employs 'like' or 'as'. So, her tears fell like rain; her lips were sweet as wine. That's a simile. And finally: 'cliché'. This is what printers used to call the plate used for stereotype printing. Now it refers to any term, phrase, or idea that's repeated so often as to lose its meaning. (You can see why 'stereotype' is now used the way it is, too.) 'Wallowing in self-pity' is an example of a cliché. The term is overused. When it's applied imprecisely to a situation, it is said to be 'trite'.
Okay? Now stoke those fires, keep your powder dry, clear the decks, and write.